Transcript Chapter 16 - Chemistry of Benzene

16. Chemistry of Benzene:
Electrophilic Aromatic
Substitution
Based on
McMurry’s Organic Chemistry, 6th edition, Chapter 16
©2003 Ronald Kluger
Department of Chemistry
University of Toronto
Substitution Reactions of Benzene and
Its Derivatives
 Benzene is aromatic: a cyclic conjugated
compound with 6  electrons
 Reactions of benzene lead to the retention of the
aromatic core
 Electrophilic aromatic substitution replaces a
proton on benzene with another electrophile
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16.1 Bromination of Aromatic Rings
 Benzene’s  electrons participate as a Lewis base in
reactions with Lewis acids
 The product is formed by loss of a proton, which is
replaced by bromine
 FeBr3 is added as a catalyst to polarize the bromine
reagent
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Addition Intermediate in Bromination
 The addition of bromine occurs in two steps
 In the first step the  electrons act as a nucleophile
toward Br2 (in a complex with FeBr3)
 This forms a cationic addition intermediate from
benzene and a bromine cation
 The intermediate is not aromatic and therefore high in
energy (see Figure 16.2)
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Formation of Product from
Intermediate
 The cationic addition
intermediate transfers a
proton to FeBr4- (from Brand FeBr3)
 This restores aromaticity
(in contrast with addition
in alkenes)
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16.2 Other Aromatic Substitutions
 The reaction with bromine involves a mechanism that
is similar to many other reactions of benzene with
electrophiles
 The cationic intermediate was first proposed by G. W.
Wheland of the University of Chicago and is often
called the Wheland intermediate
George Willard Wheland
1907-1974
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Aromatic Chlorination and Iodination
 Chlorine and iodine (but not fluorine, which is too
reactive) can produce aromatic substitution with the
addition of other reagents to promote the reaction
 Chlorination requires FeCl3
 Iodine must be oxidized to form a more powerful I+
species (with Cu+ or peroxide)
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Aromatic Nitration
 The combination of nitric acid and sulfuric acid
produces NO2+ (nitronium ion)
 The reaction with benzene produces nitrobenzene
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Aromatic Sulfonation
 Substitution of H by SO3 (sulfonation)
 Reaction with a mixture of sulfuric acid and SO3
 Reactive species is sulfur trioxide or its conjugate
acid
 Reaction occurs via Wheland intermediate and is
reversible
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Alkali Fusion of Aromatic Sulfonic
Acids
 Sulfonic acids are useful as intermediates
 Heating with NaOH at 300 ºC followed by
neutralization with acid replaces the SO3H group with
an OH
 Example is the synthesis of p-cresol
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16.3 Alkylation of Aromatic Rings: The
Friedel–Crafts Reaction
 Aromatic substitution
of a R+ for H
 Aluminum chloride
promotes the
formation of the
carbocation
 Wheland intermediate
forms
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Limitations of the Friedel-Crafts
Alkylation
 Only alkyl halides can be used (F, Cl, I, Br)
 Aryl halides and vinylic halides do not react (their
carbocations are too hard to form)
 Will not work with rings containing an amino group
substituent or a strongly electron-withdrawing group
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Control Problems
 Multiple alkylations can occur because the first
alkylation is activating
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Carbocation Rearrangements During
Alkylation
 Similar to those that occur during electrophilic
additions to alkenes
 Can involve H or alkyl shifts
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16.4 Acylation of Aromatic Rings
 Reaction of an acid chloride (RCOCl) and an
aromatic ring in the presence of AlCl3 introduces acyl
group, COR
 Benzene with acetyl chloride yields acetophenone
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Mechanism of Friedel-Crafts
Acylation
 Similar to alkylation
 Reactive electrophile: resonance-stabilized acyl
cation
 An acyl cation does not rearrange
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16.5 Substituent Effects in Aromatic
Rings
 Substituents can cause a compound to be (much) more or
(much) less reactive than benzene
 Substituents affect the orientation of the reaction – the
positional relationship is controlled
 ortho- and para-directing activators, ortho- and paradirecting deactivators, and meta-directing deactivators
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Origins of Substituent Effects
 An interplay of inductive effects and resonance
effects
 Inductive effect - withdrawal or donation of electrons
through a s bond
 Resonance effect - withdrawal or donation of
electrons through a  bond due to the overlap of a p
orbital on the substituent with a p orbital on the
aromatic ring
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Inductive Effects
 Controlled by electronegativity and the polarity of
bonds in functional groups
 Halogens, C=O, CN, and NO2 withdraw electrons
through s bond connected to ring
 Alkyl groups donate electrons
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Resonance Effects – Electron
Withdrawal
 C=O, CN, NO2 substituents withdraw electrons from
the aromatic ring by resonance
  electrons flow from the rings to the substituents
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Resonance Effects – Electron
Donation
 Halogen, OH, alkoxyl (OR), and amino substituents
donate electrons
  electrons flow from the substituents to the ring
 Effect is greatest at ortho and para
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Contrasting Effects
 Halogen, OH, OR, withdraw electrons inductively so
that they deactivate the ring
 Resonance interactions are generally weaker,
affecting orientation
 The strongest effects dominate
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16.6 An Explanation of Substituent
Effects
 Activating groups
donate electrons
to the ring,
stabilizing the
Wheland
intermediate
(carbocation)
 Deactivating
groups withdraw
electrons from the
ring, destabilizing
the Wheland
intermediate
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Ortho- and Para-Directing Activators:
Alkyl Groups
 Alkyl groups activate: direct further substitution to
positions ortho and para to themselves
 Alkyl group is most effective in the ortho and para
positions
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Ortho- and Para-Directing Activators:
OH and NH2
 Alkoxyl, and amino groups have a strong, electron-
donating resonance effect
 Most pronounced at the ortho and para positions
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Ortho- and Para-Directing
Deactivators: Halogens
 Electron-withdrawing inductive effect outweighs
weaker electron-donating resonance effect
 Resonance effect is only at the ortho and para
positions, stabilizing carbocation intermediate
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Meta-Directing Deactivators
 Inductive and resonance effects reinforce each other
 Ortho and para intermediates destabilized by
deactivation from carbocation intermediate
 Resonance cannot produce stabilization
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Summary Table: Effect of Substituents in
Aromatic Substitution
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16.7 Trisubstituted Benzenes:
Additivity of Effects
 If the directing effects of the two groups are the
same, the result is additive
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Substituents with Opposite Effects
 If the directing effects of two groups oppose each
other, the more powerful activating group decides the
principal outcome
 Usually gives mixtures of products
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Meta-Disubstituted Compounds Are
Unreactive
 The reaction site is too hindered
 To make aromatic rings with three adjacent
substituents, it is best to start with an orthodisubstituted compound
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16.8 Nucleophilic Aromatic
Substitution
 Aryl halides with
electron-withdrawing
substituents ortho and
para react with
nucleophiles
 Form addition
intermediate
(Meisenheimer
complex) that is
stabilized by electronwithdrawal
 Halide ion is lost to
give aromatic ring
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16.9 Benzyne
 Phenol is prepared on an industrial scale by
treatment of chlorobenzene with dilute aqueous
NaOH at 340°C under high pressure
 The reaction involves an elimination reaction that
gives a triple bond
 The intermediate is called benzyne
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Evidence for Benzyne as an
Intermediate
 Bromobenzene with
14C
only at C1 gives substitution
product with label scrambled between C1 and C2
 Reaction proceeds through a symmetrical
intermediate in which C1 and C2 are equivalent—
must be benzyne
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Structure of Benzyne
 Benzyne is a highly distorted alkyne
 The triple bond uses sp2-hybridized carbons, not the
usual sp
 The triple bond has one  bond formed by p–p
overlap and by weak sp2–sp2 overlap
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16.10 Oxidation of Aromatic
Compounds
 Alkyl side chains can be oxidized to CO2H by
strong reagents such as KMnO4 and Na2Cr2O7 if they
have a C-H next to the ring
 Converts an alkylbenzene into a benzoic acid, ArR
 ArCO2H
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Bromination of Alkylbenzene Side
Chains
 Reaction of an alkylbenzene with N-bromo-
succinimide (NBS) and benzoyl peroxide (radical
initiator) introduces Br into the side chain
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Mechanism of NBS (Radical) Reaction
 Abstraction of a benzylic hydrogen atom generates
an intermediate benzylic radical
 Reacts with Br2 to yield product
 Br· radical cycles back into reaction to carry chain
 Br2 produced from reaction of HBr with NBS
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16.11 Reduction of Aromatic
Compounds
 Aromatic rings are inert to catalytic hydrogenation
under conditions that reduce alkene double bonds
 Can selectively reduce an alkene double bond in the
presence of an aromatic ring
 Reduction of an aromatic ring requires more powerful
reducing conditions (high pressure or rhodium
catalysts)
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Reduction of Aryl Alkyl Ketones
 Aromatic ring activates neighboring carbonyl group
toward reduction
 Ketone is converted into an alkylbenzene by catalytic
hydrogenation over Pd catalyst
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16.12 Synthesis Strategies
 These syntheses require planning and consideration
of alternative routes
 Work through the practice problems in this section
following the general guidelines for synthesis (and
retrosynthetic analysis in 8.10)
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